1. Field of the Invention
The present invention relates to a zoom lens system.
2. Description of the Related Art
In a zoom lens system, a requirement for a high zoom ratio and a requirement for miniaturization are not compatible. For example, in a zoom lens system of two-lens-group arrangement which can be miniaturized, if lens-group moving paths are designed, i.e. a solution of the lens-group moving path is obtained, with the aim of obtaining a high zoom ratio, lens groups come into contact with each other at the long focal length extremity, or lens groups and the image plane cause interference therebetween at the short focal length extremity. On the other hand, in the case of a zoom lens system of three-lens-group arrangement, the zoom ratio can be made higher than that of a two-lens-group zoom lens system; however, miniaturization of the zoom lens system is difficult. Furthermore, if the optical power (hereinafter, power) of each lens group is determined to obtain an even higher zoom ratio, error sensitivity of each lens group becomes too high, so that a mechanism which can maintain necessary precision on the operations of the lens system under such a high sensitivity is not practical to be obtained.
It is an object of the present invention to provide a miniaturized zoom lens system having a high zoom ratio.
In order to achieve the above-mentioned object, there is provided a zoom lens system including relatively moveable more than two zooming lens groups (hereinafter, N zooming lens groups; N 2) in order to vary the focal length. Among the N zooming lens groups, an i-th (i 1, counted from the most object-side zooming lens group) zooming lens group is a switching lens group (predetermined zooming lens group) The switching lens group includes more than two sub-lens groups, and one sub-lens group is made moveable, with respect to other sub-lens groups, to either the object-side end or the image-side end.
In a short-focal-length side zooming range from the short focal length extremity to an intermediate switching focal length, the N zooming lens groups and the i-th zooming lens group are moved towards the object, while (i) distances among the lens groups are varied, and (ii) the moveable sub-lens group is being made stationary at one of the object-side end and the image-side end.
At the intermediate switching focal length, the moveable sub-lens group is moved to the other of the object-side end and the image-side end, and each of the N zooming lens groups and the i-th zooming lens group are moved towards the image plane.
In a long-focal-length side zooming range from the intermediate switching focal length to the long focal length extremity, the N zooming lens groups and the i-th zooming lens group are moved towards the object, while (i) distances among the zooming lens groups are varied, (ii) the moveable sub-lens group is being made stationary at the other end, and (iii) the zoom lens system satisfies the following condition:
mNtxe2x88x92mNw less than 0xe2x80x83xe2x80x83(1) 
wherein
mNt designates the transverse magnification of the N-th, i.e., the most image-plane side, zooming lens group at the intermediate switching focal length, under the condition that the moveable sub-lens group of the switching lens group is being made stationary at the other end; and
mNw designates the transverse magnification of the N-th, i.e., the most image-plane side, zooming lens group at the intermediate switching focal length, under the condition that the moveable sub-lens group of the switching lens group is being made stationary at the one end.
The most object-side zooming lens group can be formed as the switching lens group (i=1). In this case, the zoom lens system preferably satisfies the following condition:
0 less than (logZ1/log Z) less than 0.2xe2x80x83xe2x80x83(2) 
wherein
Z=ft/fw;
Z1=f1t/f1w;
fw designates the focal length of the entire the zoom lens system at the short focal length extremity;
ft designates the focal length of the entire the zoom lens system at the long focal length extremity;
f1w designates the focal length of the first lens group in the short-focal-length side zooming range; and
f1t designates the focal length of the first lens group in the long-focal-length side zooming range.
Furthermore, at least one zooming lens group can be provided on the object side of the switching lens group (i 2). In this case, the zoom lens system preferably satisfies the following condition:
0 less than (log Zi/log Z)xe2x88x92(log Zixe2x80x2/log Zxe2x80x2) less than 0.2xe2x80x83xe2x80x83(2xe2x80x2) 
wherein
Zi=mit/miw;
Zxe2x80x2=ftxe2x80x2/fw
Zixe2x80x2=mitxe2x80x2/miw;
ftxe2x80x2 designates the focal length of the entire the zoom lens system, at the long focal length extremity, when the moveable sub-lens group is being made stationary at the one end;
miw designates the transverse magnification of the switching lens group at the short focal length extremity;
mit designates the transverse magnification of the switching lens group at the long focal length extremity; and
mitxe2x80x2 designates the transverse magnification of the switching lens group at the long focal length extremity, when the moveable sub-lens group is being made stationary at the one end.
The zoom lens system according to the present invention can satisfy the following condition:
0.01 less than xcex94di/fw less than 0.3xe2x80x83xe2x80x83(3) 
wherein
xcex94di designates the traveling distance of the moveable sub-lens group at the intermediate switching focal length.
Still further, the switching lens group can be formed as a two-lens-group arrangement which is constituted by the moveable sub-lens group and another sub-lens group. In this case, the power of one lens group is preferably different from the other, e.g., if the power of the moveable sub-lens group is positive, the power of the other sub-lens group is negative, and vice versa.
The switching lens group can include a negative lens group and a positive lens group, in this order from the object. In this case, it is preferable that the negative lens group be the moveable sub-lens group.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-019916 (filed on Jan. 28, 2000) which is expressly incorporated herein by reference in its entirety.